Assuming Assad used the biological weapons (it's hard to believe he'd be so stupid as to provoke Europe and the US that way for the dubious benefits of such weapons but if he did), then we may have to respond just to assert our current strict disapproval of such weapons, since they are hard to control. Some kind of air strike might suffice, or a no fly zone.
The real problem is that both sides appear to be unpalatable from a western perspective. The opposition has a large jihadist element, and I wouldn't put it past them to use biological weapons against themselves (or civilians associated with other militias) just to blame Assad.
Ultimately, sometimes it's best for a nation to have its own civil war (as we did) when irreconcilable disputes exist, in order for a sense of finality and unity to arise after the fighting is over. Iraq shows what happens when outside forces simply decapitate the bad guy leadership, and the bad guy opposition are left to fight it out with the good guy opposition (such as they were) because there was no common cause in ending Saddam's reign. If the jihadists and "democratic" elements have to fight together to remove Assad, they are more likely to reach some kind of accommodation afterwards. If we just take out Assad, it's Iraq all over again. I hate to see children and other civilians die in the civil war that now exists, but the alternative may be worse.
Though 1500 people killed with chemical weapons. That's scary.
"The Ends Justify The Means"
If you believe killing a neighbourhood of people with chemical weapons will ensure final victory for your side... there are those who see it as justifiable.
In that case... its not unreasonable to speculate that a chemical weapons shell may have been stolen and fired from a rebel position.
‘This is not peace, it is an armistice for 20 years.’ (Ferdinand Foch. After the Treaty of Versailles, 1919).